text: In photosynthesis, plastocyanin functions as an electron transfer agent between cytochrome f of the cytochrome bf complex from photosystem II and P700+ from photosystem I.  Cytochrome bf complex and P700 are both membrane-bound proteins with exposed residues on the lumen-side of the thylakoid membrane of chloroplasts.  Cytochrome f acts as an electron donor while P700+ accepts electrons from reduced plastocyanin.